This study will characterize by a combination of biochemical, immunological and genetic approaches the inherited variation in activity of galactokinase (GALK) and galactose-1-phosphate uridylyltransferase (GALT) in both man and mouse. The specific aims are: (1) To characterize human variants of GALT both structurally and functionally. (2) To determine the frequency of common and rare variants of GALT in the population. (3) To assess the validity of estimating the frequency of polymorphic alleles with variant enzyme activities by statistically analyzing the distribution of GALT activities in a random population. (4) To characterize the genetics and biochemistry of variations in GALK activity among inbred strains in mice.